Just from my own experience with mine, regadless of outside temperature and how hard I'm riding the needle sits at the mark about 1/4 up from the bottom of the gauge where there is a break in the green band. When I have to run in stop and go traffic then it will run up about halfway or higher at which point there is a noticable difference in the heat coming up from the engine. That is why I'm thinking thermostat.
Again, just my
Paul